1. Field of the Invention
The present invention relates to a valve arrangement and in particular to a valve arrangement in which the operation of the valve can be automatically monitored.
2. Description of the Prior Art
Automatic monitoring of the operation of a valve, i.e., its opening and closing, is particularly useful when the valve is used in conditions wherein safety is critical or in precise control conditions. Such conditions as may exist, for example, in patient ventilator systems where the operation of valves must be precisely controlled in order to ensure that a correct dose of anaesthetic is delivered to a patient.
A valve arrangement, useable in patient ventilators, is described in U.S. Pat. No. 5,265,594 and includes a variable aperture through which fluid may flow. A resilient diaphragm closure elements is also provided which is adjustable to vary the size of the aperture in response to the action of a co-operating valve stem, part of which forms a moveable solenoid core. A sensor unit is also provided which senses the aperture size by monitoring the electrical properties of an electrical circuit that includes the solenoid and valve stem. In this way the position of the valve stem within the solenoid can be determined and hence an indication of the degree of opening of the aperture is obtained. A signal is emitted from the sensor unit indicating the aperture size or changes therein, which is used to control the current supplied to the solenoid.
A further valve arrangement, useable in fire protection sprinkler systems, is described in U.S. Pat. No. 5,584,319 and includes an optical sensor unit from which a signal is emitted indicating whether the valve is open. Light from the sensor unit is reflected from a valve stem and detected by a photodetector only when the valve stem is moved to unblock an aperture and open the valve.
Both valve arrangements operate by monitoring the position of the valve stem from which the position of a closure element, and hence the size of the aperture is inferred. In certain circumstances, for example as the valve arrangement becomes worn, the valve stem may move before any changes in the size of the valve aperture occurs and so may cause erroneous signals to be emitted from the sensor unit. Moreover, sticking of the closure element, may again cause measurements made based on the valve stem to provide erroneous signals output from the sensor unit.